WASHINGTON – An Army paratrooper unit that led the U.S. troop surge into Baghdad last year is returning home, marking a resumption of a troop drawdown that will test the durability of recent security gains.

The drawdown began in December with the departure of one brigade, numbering about 5,000 troops, dropping the overall U.S. troop level in Iraq to 158,000. A three-month lull was built into the drawdown plan, during which commanders saw insurgent violence shift from Baghdad to northern Iraq.

Although it has not yet been publicly announced, a senior military official told The Associated Press on Tuesday that the 2nd brigade, 82nd Airborne Division is heading back to Fort Bragg, N.C., in coming days and will not be replaced in the rotation. The official spoke on condition of anonymity because the movements are not completed.

That will drop the number of U.S. combat brigades in Iraq from 19 to 18, with an additional three scheduled to leave by July. Yet to be decided is whether further reductions will be made after July; President Bush on Saturday declined to promise that he will order more cuts before he leaves office in January.

A brigade usually numbers between 3,500 and 5,000 troops, depending on how it is organized.

Some military leaders have pushed for a continued drawdown beyond July, arguing that a strained Army and Marine Corps need relief after five years of combat and multiple tours of duty with long absences from home. On the other hand, they don’t want to reduce so quickly that security gains are lost.

The 2nd brigade, 82nd Airborne has been operating in northeast Baghdad since January 2007, when Bush announced he was sending 21,500 Army and Marine reinforcements as part of a revamped military strategy to tamp down sectarian violence. The reinforcements included five Army brigades and two Marine battalions.

The first unit to leave Iraq without being replaced in the rotation was the 3rd brigade, 1st Cavalry Division, which went home last December. It was not one of the five “surge” brigades. Rather, it reached the end of its 15-month tour and left without being replaced. That unit had been operating in volatile Diyala province, north of Baghdad, and when it departed U.S. commanders moved other forces into Diyala from nearby areas.

It was not clear Tuesday which of the remaining U.S. units are being moved into Baghdad to take over for the departing 2nd brigade, 82nd Airborne – or from which parts of the country those units are shifting. The paratrooper unit, known as Falcon Brigade and commanded by Col. Don Farris, has operated mostly in northeast Baghdad, including the Sadr City and Adhamiya districts.

In an interview Nov. 26 with reporters at the Pentagon, Farris said the number of attacks in his area had dropped by 75percent since May 1, 2007. He said security was terrible when his unit arrived.

“Whole-scale murder and displacement of families from both sects was alarming,” Farris said. “Al-Qaida remained organized, determined and extremely lethal, and Shiite militia death squads operated with impunity, committing horrific acts of violence against Iraqi citizens.”

In designing their drawdown plan, U.S. commanders have sought to shift forces in a way that avoids leaving gaps in key areas, while handing off more security responsibility to Iraqi forces where possible.

At a Pentagon news conference Tuesday, Army Lt. Gen. Ray Odierno, who was the No.2 U.S. commander in Iraq until he returned home in mid-February, noted that another brigade was coming out of Iraq. He did not identify the brigade.

“I think we’ll reduce another brigade and be down to 18 brigades next week,” Odierno told reporters.